Introduction

React.js is a popular JavaScript library for building user interfaces. Whether you're a beginner or an experienced developer, this React.js cheatsheet will help you quickly reference key concepts, syntax, and best practices.

Table of Contents

1. Components
2. JSX
3. Props
4. State
5. Lifecycle Methods
6. Events
7. Hooks
8. Conditional Rendering
9. Lists and Keys
10. Forms and Controlled Components
11. Context API
12. Refs
13. Styling
14. Error Handling
15. Conclusion

Components

React applications are built using components, which are reusable, self-contained UI elements. Components can be functional or class-based.

Functional Component:

function MyComponent(props) {
  return <div>Hello, {props.name}</div>;
}

JSX

JSX is a syntax extension for JavaScript used in React to describe the structure of your UI. It allows you to write HTML-like code within JavaScript.

Example:

const element = <h1>Hello, World!</h1>;

Props

Props (short for properties) are inputs to a component. They allow you to pass data from parent components to child components.

Usage:

<MyComponent name="John" />

Accessing props:

function MyComponent(props) {
  return <div>Hello, {props.name}</div>;
}

State

State is used to manage a component's internal data. It can be modified using setState, triggering a re-render.

Example:

import React, { useState } from 'react';

function Counter() {
  // Declare a state variable 'count' and a function to update it 'setCount'
  const [count, setCount] = useState(0);

  // Event handler to increment the count when a button is clicked
  const handleIncrement = () => {
    setCount(count + 1);
  };

  return (
    <div>
      <h1>Counter: {count}</h1>
      <button onClick={handleIncrement}>Increment</button>
    </div>
  );
}

export default Counter;

Events

React uses synthetic events, similar to native DOM events. Event handlers are defined using camelCase attributes.

Example:

   function handleClick() {
       console.log("Hi Mom!")
   }

  function MyButton() {
    return <button onClick={() => {handleClick()}>Click Me</button>;
  }

Hooks

Hooks are functions that allow you to use state and other React features in functional components. Common hooks include useState and useEffect.

Example:

import React, { useState, useEffect } from 'react';

function MyComponent() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    // Perform side effects
  }, [count]);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

Conditional Rendering

Conditional rendering allows you to show or hide elements based on conditions.

Example:

function MyComponent(props) {
  return (
    <div>
      {props.isLoggedIn ? <p>Welcome, {props.username}</p> : <p>Please log in</p>}
    </div>
  );
}

Lists and Keys

To render lists of items, map over the data and provide a unique key prop to each item.

Example:

function MyListComponent(props) {
  const items = props.data.map((item) => (
    <li key={item.id}>{item.name}</li>
  ));

  return <ul>{items}</ul>;
}

Context API

The Context API provides a way to pass data through the component tree without having to pass props manually at every level.

Example:

const MyContext = React.createContext();

function MyComponent() {
  return (
    <MyContext.Provider value="Hello, Context!">
      <MyChildComponent />
    </MyContext.Provider>
  );
}

function MyChildComponent() {
  const contextValue = React.useContext(MyContext);
  return <div>{contextValue}</div>;
}

Refs

Refs are used to access the DOM or React elements directly. They can be created using useRef hook.

Example:

  import { useRef } from 'react'

  function MyInput() {
  const myRef = useRef(null)
    return <input ref={myRef} />;
  }
}

Styling

You can apply styles to components using CSS classes, inline styles, or CSS-in-JS libraries like styled-components or emotion.

Example (inline styles):

const divStyle = {
  color: 'blue',
  backgroundColor: 'lightgray',
};

function MyComponent() {
  return <div style={divStyle}>Styled Div</div>;
}

Conclusion

This React.js cheatsheet provides a quick reference to essential concepts and features in React development. React is a powerful library for building user interfaces, and understanding these key concepts will help you create dynamic and responsive web applications.